1. Field of the Invention
The present invention relates to a half-toroidal-type continuously variable transmission which can be used, for example, as a transmission unit for a car transmission or a transmission for various kinds of industrial machines. Especially, the invention relates to a trunnion to be incorporated into such half-toroidal-type continuously variable transmission, and a method for working such trunnion with ease.
2. Description of the Related Art
Conventionally, it has been studied to use such a half-toroidal-type continuously variable transmission as schematically shown in FIGS. 7 and 8, for example, as a transmission unit for a car transmission. In the conventional half-toroidal-type continuously variable transmission, for example, as disclosed in JP-A-62-71465U, an input side disk 2 is supported concentrically with an input shaft 1, while an output side disk 4 is fixed to the end portion of an output shaft 3 disposed concentrically with the input shaft 1. In the interior portion of a casing in which the half-toroidal-type continuously variable transmission is stored, there are disposed trunnions 6, 6 swingable about their respective pivot shafts 5, 5 which are set at torsional positions with respect to the input and output shafts 1 and 3. That is, the center axes of the pivot shafts 5, 5 do not intersect with the center axes of the input and output shafts 1 and 3 but they are respectively present in the directions that are perpendicular to the directions of the center axes of the input and output shafts 1 and 3.
In the trunnions 6, 6, the pivot shafts 5, 5 are arranged on the outer surfaces of the two end portions thereof in such a manner that they are concentric with each other. The base end portions of displacement shafts 7, 7 are supported on the respective central portions of the trunnions 6, 6 and, by swinging the trunnions 6, 6 about their respective pivot shafts 5, 5, the inclination angles of the displacement shafts 7, 7 can be freely adjusted. On the peripheries of the displacement shafts 7, 7 respectively supported on the trunnions 6, 6, there are rotatably supported a plurality of power rollers 8, 8. And, the plurality of power rollers 8, 8 are respectively held by and between the input side and output side disks 2, 4. The two disks 2, 4 respectively include inner surfaces 2a, 4a which are disposed such that they are opposed to each other. The mutually opposed inner surfaces 2a, 4a each have a cross section formed as a concave surface which can be obtained by rotating an arc with the pivot shaft 5 as the center thereof. On the other hand, the power rollers 8, 8 respectively have peripheral surfaces 8a, 8a each of which is formed as a spherically convex surface. And, the peripheral surfaces 8a, 8a are respectively contacted with the inner surfaces 2a, 4a. 
Between the input shaft 1 and input side disk 2, there is interposed a pressure device 9 such as a loading cam. Thus, the input side disk 2 can be elastically pushed toward the output side disk 4 by the pressure device 9. The pressure device 9 comprises a cam plate 10 rotatable together with the input shaft 1, and a plurality of rollers 12, 12 (for example, four rollers) which are respectively held by a retainer 11. On one side surface (in FIGS. 7 and 8, the left side surface) of the cam plate 10, there is formed a cam surface 13 which consists of a curved surface extending in the circumferential direction of the cam plate 10; and, on the outer surface (in FIGS. 7 and 8, the right side surface) of the input side disk 2 as well, there is formed a cam surface 14 which is similar to the cam surface 13. And, the plurality of rollers 12, 12 are rotatably supported about their respective axes extending in the radial direction with respect to the center of the input shaft 1.
When the above-structured half-toroidal-type continuously variable transmission is in operation, in case where the cam plate 10 is rotated due to the rotation of the input shaft 1, the plurality of rollers 12, 12 are respectively pressed against the cam surface 14 formed on the outer surface of the input side disk 2 by the cam surface 13 of the cam plate 10. As a result of this, the input side disk 2 is pressed against the plurality of power rollers 8, 8 and, at the same time, the pair of cam surfaces 13, 14 and the plurality of rollers 12, 12 are pressed against each other, thereby causing the input side disk 2 to be rotated. And, the rotation of the input side disk 2 is transmitted to the output side disk 4 through the plurality of power rollers 8, 8, with the result that the output shaft 3 fixed to the output side disk 4 can be rotated.
Of cases in which the rotation speed between the input shaft 1 and output shaft 3 is to be changed, at first, in case where deceleration is carried out between the input shaft 1 and output shaft 3, the trunnions 6, 6 are swung about their respective pivot shafts 5, 5, and the displacement shafts 7, 7 are inclined in such a manner that the peripheral surfaces 8a, 8a of the power rollers 8, 8, as shown in FIG. 7, can be respectively contacted with the near-center portion of the inner surface 2a of the input side disk 2 and with the near-outer-periphery portion of the inner surface 4a of the output side disk 4. On the other hand, in the case of acceleration, the trunnions 6, 6 are swung about their respective pivot shafts 5, 5, and the displacement shafts 7, 7 are inclined in such a manner that the peripheral surfaces 8a, 8a of the power rollers 8, 8, as shown in FIG. 8, can be respectively contacted with the near-outer-periphery portion of the inner surface 2a of the input side disk 2 and with the near-center portion of the inner surface 4a of the output side disk 4. In case where the inclination angles of the displacement shafts 7, 7 are set in the middle of the inclination angles in FIGS. 7 and 8, there can be obtained a middle transmission ratio.
The specific shapes of the trunnions 6, 6 as well as the structures of portions for supporting the power rollers 8, 8 on the trunnions 6, 6 are conventionally known; that is, they are disclosed, for example, in JP-A-8-240251. Now, FIGS. 9 to 11 shows a conventional structure of a portion for supporting the power roller 8 on the trunnion 6. In the middle portion of the trunnion 6, there is formed a circular hole 15. The circular hole 15 is formed in a direction perpendicular to the pivot shafts 5, 5 respectively disposed on the two end portions of the trunnion 6 (that is, in a direction where the extensions of the center axes of the pivot shafts 5, 5 and the extension of the center axis of the circular hole 15 are perpendicular to each other). And, on the portion of the trunnion 6 that is present inside the circular hole 15, there is supported the displacement shaft 7. The displacement shaft 7 includes a support shaft portion 16 and a pivot shaft portion 17 which are arranged in parallel to each other but are offset from each other in the axes thereof. Of these two portions of the displacement shaft 7, the support shaft portion 16 is rotatably supported inside the circular hole 5 through a radial needle roller bearing 18. Also, on the periphery of the pivot shaft portion 17, there is rotatably supported the power roller 8 through another radial needle roller bearing 19.
Also, between the outer surface of the power roller 8 and the inner surface of the middle portion of the trunnion 6, there are interposed a thrust ball bearing 20 and a thrust needle roller bearing 21 in order starting from the outer surface of the power roller 8. Of these two bearings, the thrust ball bearing 20, while supporting a thrust load applied to the power roller 8, allows the power roller 8 to be rotated. The thrust ball bearing 20 is composed of a plurality of balls 22, 22, an annular-shaped retainer 23 for rollably holding the balls 22, 22, and an annular-shaped thrust ball bearing outer race 24. The inner race raceways of the respective thrust ball bearings 20 are formed on the outer surface of the power roller 8, while the outer race raceways of the respective thrust ball bearings 20 are formed on the inner surface of the thrust ball bearing outer race 24.
Also, the thrust needle roller bearing 21 is composed of a race 25, a retainer 26 and needle rollers 27, 27. The thrust needle roller bearing 21 is held by and between the inner surface of the trunnion 6 and the outer surface of the thrust ball bearing outer race 24, with the race 25 contacted with the inner surface of the trunnion 6. On the other hand, a cylindrical-shaped radial needle roller bearing outer race 28, which forms a part of the radial needle roller bearing 18, is fitted with and fixed to the inner side of the circular hole 15 with no play between them. And, in the thus fitted and fixed state, one end portion of the radial needle roller bearing outer race 28 is projected from the inner surface of the trunnion 6, and the race 25 and retainer 26, which respectively form the thrust needle roller bearing 21, are fitted with the outer surface of the projecting portion of the radial needle roller bearing outer race 28, thereby positioning the race 25 and retainer 26 with respect to the trunnion 6.
The thus structured thrust needle roller bearing 21, while supporting a thrust load applied to the thrust ball bearing outer race 24 from the power roller 8, allows the pivotal shaft portion 17 and thrust ball bearing outer race 24 to be swung about the support shaft portion 16. That is, in the half-toroidal-type continuously variable transmission, due to its transmission operation, variations in the torque to be transmitted and the elastic deformation of the component members of the half-toroidal-type continuously variable transmission, one or both of the input side and output side disks 2, 4 (FIGS. 7 and 8) are caused to shift in the axial direction, so that the displacement shaft 7 is slightly rotated about the support shaft portion 16. As a result of the slight rotation of the displacement shaft 7, the outer surface of the thrust ball bearing outer race 24 of the thrust ball bearing 20 and the inner surface of the trunnion 6 are shifted with respect to each other. Since the thrust needle roller bearing 21 is present between these outer and inner surfaces, such relative shift can be attained with only a small force. Therefore, there is required only a small force to change the inclination angles of the displacement shafts 7, 7 in the above-mentioned manner.
In the case of the trunnion 6 employed in the above-structured half-toroidal-type continuously variable transmission, a blank member formed of metal having high rigidity such a carbon steel is forged to be worked into a given shape and, after then, the required portions of the thus worked member are cut or ground for finishing. Of the respective portions of the thus worked trunnion 6, the circular circle 15 for assembly of the radial needle roller bearing 18 and a flat surface 29 for additional attachment of the thrust needle roller bearing 21 are respectively formed by a cutting operation using a machining center. For example, to machine the flat surface 29, with a rotation tool pressed against the inner surface middle portion of the trunnion 6 to be formed as the flat surface 29, the rotation tool may be rotated, with the result that the flat surface 29 can be worked. In a conventional machining operation, the cutting operation of the flat surface 29 is carried out separately from the drilling and finishing cutting operations of the circular hole 15. That is, after one of the flat surface 29 and circular hole 15 is worked by a machine tool, the trunnion 6 is once removed from the machine tool and is then set in a second machine tool, and the other of the flat surface 29 and circular hole 15 is then machined by the second machine tool.
To work the trunnion 6 for the half-toroidal-type continuously variable transmission with high efficiency and high accuracy, preferably, the flat surface 29 and circular hole 15 may be worked in one step which eliminates the need to remove the trunnion 6 from one machine tool and set the trunnion 6 in another machine tool.
The present invention aims at eliminating the above-mentioned drawbacks found in the conventional trunnion for use in a half-toroidal-type continuously variable transmission. Accordingly, it is an object of the invention to provide a trunnion for use in a half-toroidal-type continuously variable transmission and a method for working such trunnion.
In attaining the above object, according to one aspect of the invention, there is provided a trunnion of a half-toroidal-type continuously variable transmission, comprising: a middle portion having a flat surface on an inner surface thereof; and a pair of pivot shafts respectively disposed on the both end portions of the trunnion and being concentric with each other. A circular hole is formed in a art of the flat surface, with one end thereof opened. In the flat surface, there are formed stepped surfaces in the boundary portions between the peripheral edge portions of the flat surface and the remaining portion thereof, and the stepped surfaces respectively define partially tubular surfaces having a single center axis. The single center axis of the partially tubular surfaces is located at a position offset from a center axis of the circular hole in a direction in which the pair of pivot shafts are arranged.
According to a second aspect of the invention, there is provided a method for working a trunnion for a half-toroidal-type continuously variable transmission. The trunnion comprises: a middle portion having a flat surface formed on an inner surface thereof; and a pair of pivot shafts respectively disposed on the two end portions of the trunnion and being concentric with each other. A circular hole is formed in a part of said flat surface, with one end thereof opened. In the flat surface, there are formed stepped surfaces in the boundary portions between the peripheral edge portions of the flat surface and the remaining portion thereof, and the stepped surfaces respectively define partially tubular surfaces having a single center axis. The single center axis of the partially tubular surfaces is located at a position offset from a center axis of said circular hole in a direction in which said pair of pivot shafts are arranged. The method, comprises the steps of: holding the trunnion by a chuck supported on the leading end portion of a spindle of a machine tool performing a cutting operation in such a manner that the center axis of one of the circular hole and the partially tubular surfaces is coincident with or parallel to the center axis of the spindle; working one of the circular hole, the partially tubular surfaces and the flat surface of the held trunnion, while rotating the spindle; moving the chuck in a direction which is perpendicular to the center axis of the spindle and is parallel with the arranged direction of the pivot shafts of the trunnion; and, working the other of the circular hole, the partially tubular surfaces and the flat surface of the trunnion, while rotating the spindle.
In a trunnion for a half-toroidal-type continuously variable transmission and a method for working such trunnion structured in the above-mentioned manner according to the invention, while the trunnion remains held on and by the same chuck, the circular hole and partially tubular surfaces as well as the flat surface can be worked, thereby being able to enhance the working efficiency of the trunnion. This not only can reduce the manufacturing cost of the trunnion but also can enhance the shape and dimension accuracy of the trunnion such as the perpendicularity between the circular hole and flat surface.